1. Field of the Invention
This invention relates generally to a miniature motor, and more particularly to a miniature motor driven by a full-wave or half-wave rectified power source and using permanent magnets as magnetic poles. In the invention, noise-suppressing capacitors are mounted on the rotor side, and a provision is made to minimize the unwanted shaft wobbling of the motor shaft.
2. Description of the Prior Art
In general, a miniature motor has a stator having a pair of permanent magnets and a rotor having a rotor winding (not shown) wound on a three-pole rotor core, as shown in FIG. 5. FIG. 5 is a cross-sectional view of a miniature motor of a conventional type. In FIG. 5 reference numeral 1 denotes a motor case; 2 a permanent magnet; 3 a rotor core; 3-1 through 3-3 core arms; 4 a motor shaft; 6 a magnet stopper; and 7 a magnet retaining spring, respectively.
The prior-art miniature motor shown in FIG. 5 has a rotor core 3 of a three-pole construction. The outer circumferential surface each of the core arms 3-1 through 3-3 of the rotor core 3 is formed into a substantially circular arc shape so that the gap between the core arm circumferential surface and the permanent magnets 2 is kept almost uniform. Both ends of a motor shaft 4 are supported by bearings (not shown). By allowing electric current to flow in the rotor winding (not shown) wound on each of the core arms 3-1 through 3-3, the rotor core 3 placed in a field formed by the permanent magnets 2 is caused to rotate.
In the miniature motor of the conventional type shown in FIG. 5, a magnetic attraction force is exercised between the permanent magnets 2 and the rotor core 3. With the rotor core having a three-pole construction, however, the attraction force tends to be asymmetrical with respect to the motor shaft 4. As a result, the balance of the magnetic attraction force generated between the permanent magnets 2 and the rotor core 3 is disturbed, with the result that the wobbling of the rotating motor shaft in the bearings is increased, leading to increased wobbling noise. In the following, the prior art will be described, referring to FIGS. 6 (I) and (II) through FIG. 8.
FIG. 6 is a diagram of assistance in explaining the magnetic attraction force produced between the permanent magnets and the rotor core in the prior art shown in FIG. 5. FIG. 6 (I) shows the case where voltage is zero for a motor that is caused to rotate by applying half-wave rectified voltage (hereinafter referred to as the case where half-wave rectified voltage is OFF). FIG. 6 (II) shows the case where a voltage is applied to the motor in a half-wave rectified voltage state (hereinafter referred to as the case where half-wave rectified wave is ON). FIGS. 7 (A) and (B) are photos of oscilloscope waveforms representing vibration measurements, and FIG. 8 is a diagram of assistance in explaining vibration measuring tests. Although the permanent magnet 2 in FIGS. 6 (I) and (II) is of a ring shape, the same effects can be achieved with arc-shaped permanent magnets as shown in FIG. 5.
In FIG. 6 (I), (1A) through (4A) represent the states of the rotor core 3 at each 30 degree angular position as the rotor core 3 is caused to rotate 90 degrees. FIG. 6(I) (1B) and (4B) show the forces exerted on the motor shaft 4 at each of these angular position (hereinafter referred to as the shaft radial forces. The sizes of the arrows, however, indicate the magnitudes of the forces only schematically.). In FIG. 6 (I) where no electric current is fed to the rotor winding 8, the force exerting on the shaft (the shaft radial force) can be considered attributable to the attraction force by the permanent magnet 2 to the motor core arms, if residual magnetism in the core arms 3-1 through 3-3 of the rotor core 3 is neglected. The shaft radial forces at the angular position shown in (1A) are balanced in both the X and Y directions, as shown in (1B). The shaft radial forces at the angular position shown in (2A), in which the rotor core 3 is turned 30 degrees from the position shown in (1A), are balanced in the X direction, but not in the Y direction, as shown in (2B). The shaft radial forces at the angular position shown in (3A), in which the rotor core 3 is further turned 30 degrees, are balanced in both the X and Y directions, as shown in (3B). Furthermore, the shaft radial forces at the angular position shown in (4A), in which the rotor core 3 is further turned 30 degrees, are balanced in the X direction, but not in the Y direction, as shown in (4B). In the foregoing, changes in shaft radial forces as the rotor core 3 is turned 90 degrees have been described. The changes in shaft radial forces at other angular positions are the repetition of the states shown in (1B) through (4B) above. That is, every time the rotor core 3 is turned 30 degrees, the imbalance of the shaft radial forces takes place, and the wobbling of the rotating motor shaft 4 is increased, leading to wobbling noise.
FIG. 6 (II), (1A) through (4A) represent the states of the rotor core 3 at each 30-degree angular position during the period in which the rotor core 3 is turned 90 degrees, while (1B) through (4B) show the shaft radial forces at each angular position. FIG. 6 (II) shows the state where half-wave rectified voltage is ON. As is well known, the current is fed to the rotor winding 8 via the brushes (not shown) and the commutator. Consequently, the manner in which current is fed to the rotor winding 8 changes in accordance with the changes in angular position of the rotor core 3, as shown in (1A) through (4A) in FIG. 6 (II). Since the core arms 3-1 through 3-3 are magnetized by the electric current flowing the rotor winding 8, the shaft radial forces rely on the relative attraction or repulsion force between the permanent magnet 2 and core arms 3-1 through 3-3.
In FIG. 6 (II), the shaft radial forces at the angular position shown in (1A) are unbalanced, as shown in (1B). The shaft radial force at the angular position shown in (2A), in which the rotor core 3 is turned 30 degrees from the state shown in (1A) are as shown in (2B). The shaft radial forces at the angular position shown in (3A), in which the rotor core 3 is further turned 30 degrees, are unbalanced, as shown in (3B). The shaft radial forces at the angular position shown in (4A), in which the rotor core 3 is still further turned 30 degrees, are as shown in (4B). In the foregoing, changes in shaft radial forces as the rotor core 3 is turned 90 degrees have been described. Shaft radial forces at other angular positions are the repetition of the states shown in (1B) through (4B). That is, the imbalance of shaft radial forces occurs every time the rotor core 3 is turned 30 degrees, and the wobbling of the rotating motor shaft 4 is increased, resulting in wobbling noise. Changes in shaft radial forces are similar in the case where a full-wave rectified voltage is fed to the rotor winding 8.
In the foregoing, the mechanism of the generation of unbalanced shaft radial forces that is responsible for wobbling noise in the prior-art miniature motor having a three-pole rotor core has been described. FIGS. 7 (A) and (B) show the results of vibration measurements in the prior-art miniature motor, which were obtained with the test setup shown in FIG. 8. The test setup was such that a miniature motor 10 was mounted on a resonator box 12 rested on a cushioning member 11, such as a sponge. The vibration waveforms of the miniature motor 10 were measured on an oscilloscope 14 via a vibration pickup 13 mounted on the resonator box 12. Test conditions were such that a fan 15 was used as a load to the miniature motor 10, and the revolution of the motor was 10,000 rpm.
FIGS. 7 (A) and (B) are photos of oscilloscope waveforms representing vibration measurements measured on a prior-art miniature motor (as shown in FIG. 5) under the above test conditions. FIG. 7 (A) shows the measurements in the case where source voltage is a half-wave rectified voltage, while FIG. 7 (B) shows those in the case where source voltage is a full-wave rectified voltage. In both figures, the waveform shown in the upper part of the figure is the measured vibration waveform, and that shown in the lower part is the source voltage waveform fed to the motor. Parallel dot lines in FIGS. 7 (A) and (B) indicate the maximum and minimum values of the oscilloscope waveforms. As is apparent from FIGS. 7 (A) and (B), the vibration (imbalance of shaft radial forces) responsible for wobbling noise in prior-art miniature motors is considerably large, particularly with half-wave rectified source voltage (FIG. 7 (A)). With the half-wave rectified source voltage involving repeated on-off cycles, the rotor core 3 is heavily excited under the influence of rush current. This leads to much higher wobbling noise than with the full-wave rectified source voltage. As is evident from FIGS. 7 (A) and (B), the peak values of vibration appear corresponding to the cycles of the source voltage.
To solve the above-mentioned problem, various means have been considered, including controlling of the type and volume of lubricant in the bearings, and adjusting the clearance between the motor shaft and the bearings. As noted earlier, the intense wobbling of the rotating motor shaft tends to accelerate the wear of the bearings, and increase the clearance between the motor shaft and the bearings, leading to increased wobbling noise. In other cases, this results in burn-out and sticking between the motor shaft and the bearings.
To solve the aforementioned problems, the present Applicant had proposed a miniature motor in a previous Utility Model Application No. 94445/1987 shown in FIG. 9. The previous proposal shown in FIG. 9 was intended to prevent wobbling noise from generating in the bearings by providing core cut portions 3-1' through 3-3' formed by cutting the outer circumferential surfaces of the core arms 3-1 through 3-3 in a three-pole rotor core 3, and making the gap over a predetermined range at the central part of the outer circumferential surface of the rotor core 3 facing the permanent magnet 2 larger than the gap over other ranges to maintain the balance of magnetic attraction force generated between the permanent magnet 2 and the rotor core 3 over the entire circumference of the rotating rotor core 3, thereby preventing the unwanted wobbling of the motor shaft during motor rotation. Photos of oscilloscope waveforms representing the results of vibration measurement tests conducted with a test setup similar to that shown in FIG. 8 are shown in FIGS. 10 (A) and (B). According to the test results, it cannot necessarily be said that the previous proposal yields satisfactory effects. FIG. 10 (A) represents the case in which source voltage was a half-wave rectified voltage, while FIG. 10 (B) represents the case in which source voltage was a full-wave rectified voltage.
Furthermore, the present Inventor conducted tests by controlling the phase angle of the brushes, but was not able to achieve the desired effects. FIGS. 11 (A) and (B) are photos of oscilloscope waveforms representing the results of vibration measurement tests conducted with a test setup similar to that shown in FIG. 8. FIG. 11 (A) represents the case where the phase angle of the brushes was controlled at a lead angle of 15 degrees, while FIG. 11 (B) represents the case where the phase angle was controlled at a lag angle of 15 degrees.
In addition, the installation of noise-suppressing capacitors poses a problem in reducing the size of miniature motors since incorporating noise-suppressing capacitors in a miniature motor would increase the size of the motor.